Radio direction finder



March 15, 1938.

E. D. BLODGETT RADIO DIRECTION FINDER Filed Dec. 51, 1935 Patented Mar.15, 1938 UNETED STATES PATENT QFFIQE RADIO DIRECTION FINDER tion ofDelaware Application December 31, 1935, Serial No. 56,861

5 Claims.

My invention relates to radio direction finders. More specifically, myinvention is a multi-frequency range radio direction finder in which aplurality of inductors are arranged for tuning with single variablecapacitors. A suitable multi-pole, multi-position switch is used tosimultaneously connect the several inductors of a frequency range totheir respective tuning capacitors.

It has been the practice to tune the antenna of a radio directionfindersystem with a variable inductor when the system is used fordetermining the sense of direction. The loop may be resonated by avariable capacitor and is rotated for determining the bearing with theantenna detuned. There are several drawbacks to the variable inductortuning: one, it is. bulky; two, it is expensive; three, the inductordoes not track with a variable capacitor; four, a multiplicity ofvariable inductors is required in a multi-range instrument; and live,the impedance characteristic .is not suited to the balance of thesystem.

I propose to overcome these difficulties and to gain certain advantagesin performance by the novel means and arrangement I am about todisclose. One of the objects of my invention is embodied in the designof a multi-frequency range radio direction finder.

Another object is in an arrangement in which all of the tuning iseffected by variable capacitors.

A further object is in the novel switching and detuning means which areembodied in my invention.

A still further object is in the design of a direction finder system inwhich the impedance characteristic of the antenna system is suitablyrelated to the associated circuits and apparatus.

Additional objects will appear in the accompanying specification, claimsand drawing to which reference is made for a proper understanding of myinvention.

An antenna I is connected through a resistor 3 to ground. A trimmercapacitor 5 is connected across. the resistor 3. A small capacitor Icouples the antenna to the movable contactor 9 of theswitch section II,and to the stator of tuning capacitor I3. The rotor of this capacitor isgrounded.

Three inductors I5, I1, and I 9.01" suitable inductance values areconnected to the fixed terminals of the switch section ii, and to thefixed terminals of a second switch section 2 I. The movable contactmember 23 of this switch section 2| is connected to the lower left handterminal of a double pole, double throw switch 25. The upper left handand lower right hand terminal of the switch 25 are connected to ground.

Each of the three inductors I5, I! and I9 are tapped intermediate theirends. The taps of each inductor are connected through suitable variableresistors 21, 29, 3| to the fixed terminals of a third switch section33. The movable contact element of this switch section 33 is connectedto the upper right hand terminal of the double pole, double throw switch25. Trimmer capacitors 35, 31, 39 are connected, respectively, betweenthe upper terminal of each of the inductors l5, l1, l9 and ground.

The portions of the inductors between their upper ends and the taps arechosen so that each may be tuned throughout the required frequency rangeby the single variable capacitor I3. The

portion below the tap is chosen to have the required detuning effectwhen the receiver is operated on bearing balance as will be describedbelow. The several resistors are of such values as will give therequired broadness of tuning of the antenna circuit.

The upper movable blade of the double pole, double throw switch isconnected to the primary M of a sense transformer 43. The primarywinding is grounded and suitably shielded from its secondary by a shield45. The lower movable blade of the double pole, double throw switch 25is connected to the primary 4'! of a balance or bearing transformer 49.The primary is grounded and shielded from its secondary by a conductor5!. The coupling between the primary 4! and the secondary is variable.

The loop circuit consists of the loop 53. The terminals of the loop areconnected to terminals 55-57. One of these terminals 55 is connected tothe serially connected loading coils 59, BI. The lower coil is connectedto the secondary 53 of transformer 43. The remaining terminal of thesecondary is connected to the variable capacitor and to the grid ofradio frequency amplifier 91.

The remaining terminal of the loop circuit 51 is connected to theconnected loading coils 61, 69. The lower coil 69is connected to thesecondary II of the balance transformer 49. The upper terminal of thissecondary II is connected to the remaining terminal of the variabletuning capacitor 65. A small trimmer capacitor I9 is connected betweenthe tuning capacitor and ground to balance the grid-cathode capacitor ofthe radio frequency amplifier tube 91.

The several frequency ranges of the loop circuit are obtained byselectively varying the insuitable audio frequency amplifier I33.

ductance included in the loop circuit. One method of varying theinductance is to use both loading coils on each side of the loop circuitin series, using one and shorting the other load coil on each side ofthe loop circuit, or shorting both loading coils on each side of theloop circuit. The switching just described, is accomplished by foursections of three-way switches, l3, l5, ll, I9.

In ordinary practice, it is desirable to employ trimmer capacitors toline up the several tuning ranges of the loop circuit. One trimmercapacitor BI is connected across the loop circuit. Three additionaltrimmer capacitors 33, 85, ill are suitably connected by a pair ofswitch sections 89-4)! so that each frequency range is provided with atrimmer capacitor. It should be understood that the trimmer capacitorsin the antenna, loop, and other circuits of this application are foralignment at the high frequency ends of each tuning range. The mainvariable tuning capacitor 65 of the loop circuit does not employ agrounded rotor.

Although numerous arrangements of amplifier and detector circuits may beconnected to the loop circuit, I prefer a superheterodyne preceded by aradio frequency amplifier. The radio frequency stage is represented bythe numeral $33. The control grid 95 of a radio frequency amplifier tube9? is connected to one terminal of the variable tuning capacitor 65. Thecathode S9 is connected through a self-biasing resistor IEll to ground.This resistor may be by-passed. The screen grid I03 is connected to asource of positive biasing potential. A suppressor grid iiit is shownconnected to the cathode 99.

The anode Ill! of radio frequency amplifier tube 81' is connectedthrough suitable coupling means to the first detector 869. The anode isalso connected to a variable tuning capacitor III. The rotor of thiscapacitor is preferably grounded. The anode is also connected to themovable contact MB of a switch section IE5. The three fixed contacts ofthis switch section H5 are connected to three inductors H7, H9, IZI. Thelower terminals of these inductors are connected to each other and tothe movable contact of a switch section I23. The fixed contacts of thisswitch section I23 are connected to the inductors so that an inductornot in use, but whose natural frequency falls within the tuning range inuse, may be short circuited to avoid undesirable coupling effects.

Each of these inductors may be shunted with a trimmer capacitor foralignment at the high frequency ends of their tuning ranges. The commonterminal of these conductors is connected through a bypass capacitor I25to ground and to a positive terminal of the 3 supply circuit.

The radio frequency amplifier 93 is coupled to the first detector I09,as explained above. The first detector I09 is coupled to an intermediatefrequency amplifier I27 through suitable intermediate frequencytransformers or the like. The output of the intermediate frequencyamplifier I2? is impressed on the second detector I29. A localintermediate frequency oscillator Isl may be used for heterodyne rception. The output of the second detector IZQ may be amplified by a Theoutput of this amplifier is represented by the numeral I which may betelephone receivers, meters .or the like. The power supply for theheaters, biases, screen grids and anodes is represented by the numeralI31. The foregoing elements are so well known to those skilled in theart, and

not being a part of my invention per se, have not been shown.

The first detector IDS is supplied with local oscillatory currents whichcombine or mix with the incoming signal representing currents to formintermediate frequency currents. The local oscillations are generated inthe conventional manner by a thermionic tube I39. The control grid MI ofthis tube is connected through grid leak resistor I53 to ground, andthrough grid capacitor M5 to the stator of a variable tuning capacitorI41. The rotor of this capacitor is grounded.

The stator of the tuning capacitor I ll is connected to the movableelement of a switch section I 49. The fixed switch contacts are,respectively, connected through oscillator padding capacitors IEI, I53,I55 to the upper terminals of inductors I51, I59, Ifil. The paddingcapacitors I51, I59, and I6I may be shunted with trimmer capacitors i63,I64, and 35. The lower terminals of the three inductors I51, I59, IISIare connected to ground. Trimmer capacitors I56, I67, I68 are connectedfrom the fixed contacts of switch section Hill to ground.

Taps intermediate the ends of the inductors I57, I59, lfil are connectedto the fixed contacts of a switch section I69. The movable contact I'IIof this switch section is connected to the cathode N3 of the oscillatortube I39. Two switch sections H5, II'I are suitably connected to theoscillator inductors so that an unused inductor, whose natural frequencyof oscillation would fall within the range of the inductor in use, maybe short circuited to avoid the deleterious eifects of coupling betweencircuits tuned to the same frequency.

The suppressor grid I'i9 of the oscillator tube 538 is connected toground. The screen grid IBI is connected through resistor !83 to asuitable positive biasing potential. The anode I84 of the oscillator isconnected to a screen grid electrode in the first detector we. Thepositive potential for the anode is derived through a resistor I85.

The several switch sections which are employed in the antenna, loops,radio frequency amplifier, and local oscillator circuits are connectedby a common shaft I81. A single control knob I89 connected to the commonshaft may be used to simultaneously operate all the switch sections. Therotors of the variable tuning capacitors in the antenna, loop, radiofrequency, and local oscillator are likewise mounted on a common shaftIQI. A single control knob I93 connected to the shaft ISI simultaneouslyvaries the several rotors for tuning to resonance,

Having described one embodiment of my invention, I shall now describethe operation. It should be understood that the frequency range of adirection finder is determined by the frequency of the transmitterswhose direction is to be determined. The receiver frequency band havingbeen determined for a particular transmitter, the double pole, doublethrow switch 25 is first turned to the left. In this position, theprimary of the sense transformer 43 is grounded. The inductor of therequired frequency range is connected with its maximum inductance in theantenna circuit. This detunes the antenna and eliminates the efiect ofthe sense direction through the ungrounded transformer 49.

The frequency knob I93 is varied until the receiver including the loopcircuit, is tuned to resonance with the incoming signal. The loopcircuit being symmetrically arranged, has a sensitween antenna and loopcircuit currents.

tivity characteristic of a figure 8 with respect to the magnetic fieldradiated by the transmitter under observation. The loop is rotated aboutits vertical axis until a minimum or null signal response is obtained.The position of minimum response is that in which the plane of the loopis normal to the direction of the transmitter. The bearing of thetransmitter is obviously on a line at right angles to the plane of theloop.

Thus having determined the bearing, it is now desirable to determine thesense of direction of the transmitter. The double pole, double throwswitch 25 is now thrown toward the right. In this position, the primaryof the bearing or balance transformer 49 is grounded. The antennacircuit is completed through the proper resistor to the tap on theinductor. The tap insures that the antenna circuit, already tuned by thecapacitor is resonant to the required frequency. The resistor insuresthat the resonant point of the antenna is not too sharp for suitabletracking with the other tuned circuits, and that the proper amount ofenergy is fed to the loop circuit. The double throw, double pole switchconnects the antenna circuit to the loop circuit through the sensetransformer 43.

The loop may now be rotated about its vertical axis to indicate thesense of the direction of the transmitter. That is, the maximum signalwill indicate that the transmitter lies along the previously determinedbearing, and in the direction of the maximum response. The position ofmaximum response for a given installation may be determined with respectto a known side of the loop by calibration with respect to stations ofknown directions.

As previously indicated, the use of a variable inductor in the antennacircuit is not desirable because the tracking characteristic of avariable inductance and a circuit tuned with an ordinary variablecapacitor are far from similar. With capacitors having similar rates ofvariation in the several tunable circuits, inductors of the properlyrelated values may be used to insure substantially perfect tracking.

When the switches are in position for the balance or bearing indication,the antenna circuit is preferably tuned to a frequency about 15% lowerthan the incoming signal frequency. This detuning provides the requiredphase relation be- It should be understood that by appropriate changesthe antenna inductors could be tuned to frequencies 15% higher insteadof lower. These percentages are given merely by way of example as theexact percentage will depend upon the characteristics of the circuitsemployed.

When operating the sense indication, the resistors serve two purposes:one, broadening the resonant point; two, controlling the amplitude ofthe currents of the antenna circuit. Normally, the ratio of antennaenergy to loop energy required for the bearing balance is greater at thehigh frequency end of a range of frequencies than at the low frequencyend of the range.

Since the effective height of the loop increases with increasingfrequency more rapidly than the effective height of the antenna, it isdesirable to increase the transfer of energy from the antenna circuit tothe loop circuit at the higher frequencies in a given band to maintain asubstantially uniform ratio of balance energy throughout the band. Bythe proper choice of inductor and trimmer capacitor values, the detuningfor resonance is made less at the high frequency end of the hand than atthe low frequency end. This adjustment will give the required energyratios.

By way of example, I have found a direction finder embodying thefollowing frequency bands or ranges very useful in practice: A range1500 to 540 kilocycles, B range 600 to 225 kilocycles, and C range 250to 100 kilocycles. It should be understood that more or less frequencyranges may be employed. Likewise, a greater or lesser range offrequencies within a given band may be used.

I do not limit my invention to the precise switching means shown whichis merely illustrative. The several inductors may be separatelyshielded, separately trimmed, and switched into or out of the circuitsas a whole. Although it is usually more convenient to operate a singleknob to switch to any of the frequency bands, separate switching in theseveral circuits may be employed.

Numerous modifications within the scope of my invention will occur tothose skilled in the art. My invention is not to be limited to theprecise embodiment shown and described, but is only limited as requiredby the prior art and appended claims.

I claim as my invention:

1. In a radio direction finder, a tunable loop circuit comprising afixed loop inductor and variable capacitor, a tunable antenna circuitcomprising an inductor and variable capacitor, a pair of transformersconnecting said loop and antenna circuits, means for simultaneouslyvarying said variable capacitors, means for changing the value of saidinductor thereby detuning said antenna circuit and means for couplingthrough one of said transformers for determining the bearing of atransmitter by balancing the currents created by the radiation of saidtransmitter in said loop circuit, means for regulating the energytransferred from said antenna circuit and means for coupling through theother of said transformers for determining the sense of direction ofsaid transmitter, and means for short circuiting the primary of thetransformer which is not in use when either bearing or sense directionsare being determined.

2. In a multi-frequency range radio direction finder, a tunable loopcircuit comprising a fixed loop inductor, a variable capacitor, and aplwrality of loading inductors; means for selectively switching saidloading inductors with respect to said loop circuit; tunable antennacircuit com.- prising a plurality of loading inductors and a variablecapacitor; means for selectively switching said loading inductors withrespect to said antenna circuit; a common control for said variablecapacitors; a second common control for said switching means; a sensetransformer having its secondary in said loop circuit; a balancetransformer having its secondary in said loop circuit; means fordetuning said antenna circuit during bearing determination; means fortuning said antenna circuit during sense of direction determination;means for substantially eliminating the effect of said sense transformerduring bearing determination, and means for substantially eliminatingthe efiect of said balance transformer during sense of directiondetermination.

3. In a multi-frequency range radio direction finder, a tunable loopcircuit comprising a fixed loop inductor, a variable capacitor, and aplurality of loading inductors; means for selectively switching saidloading inductors with respect to said loop circuit; tunable antennacircuit comprising a plurality of loading inductors and a variablecapacitor; means for selectively switching said loading inductors withrespect to said antenna circuit; a common control for said variablecapacitors; a second common control for said switching means; a sensetransformer having its secondary in said loop circuit; a balancetransformer having its secondary in said loop circuit; means fordetuning said antenna circuit during bearing determination; means fortuning said antenna circuit during sense of direction determination;means for substantially eliminating the effect of said sense transformerduring bearing determination, means for substantially eliminating theeffect of said balance transformer during sense of directiondetermination, and means for detuning said antenna a smaller percentageof frequency at the higher frequencies than at the lower frequencies ofthe range.

4. In a device of the character of claim 2, means for broadening thetuning of said antenna circuit when said direction finder is operated todetermine sense of direction.

5. In a device of the character of claim 3, means for broadening thetuning of said antenna circuit when said direction finder is operated todetermine sense of direction.

EDWARD D. BLODGETT.

